24. Petrography and Geochemistry of Red Sea Dolomite
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24. PETROGRAPHY AND GEOCHEMISTRY OF RED SEA DOLOMITE Peter R. Supko,1 Peter Stoffers,2 and Tyler B. Copied INTRODUCTION in the Site Report section of this volume and are further discussed by Stoffers and Ross (this volume). Generalized Examination of piston cores and, particularly, sediments stratigraphic columns with unit correlations are shown in collected by the Deep Sea Drilling Project has shown that Figure 2 and unit boundary depths, in terms of both meters authigenic dolomite occurs in a wide range of sedimentary subbottom and core numbers, are given in Table 1. associations. While the trace to a small percentage of Unit I—This unit is basically a biogenic calcareous ooze dolomite often encountered in pelagic carbonate sequences with varying admixtures of detrital silt and clay. may be a result of authigenesis under "normal" deep-sea Nannofossil platelets and foraminifera commonly make up conditions, the few studies to date seem to indicate that the calcareous fraction, with a significant (10%-30%) dolomite is abundant only in sediments presumably formed addition of micarb4 which, in the Red Sea, is thought to under conditions other than normal open marine. For mostly represent diagenetically altered coccolith material. example, certain dolomite sequences have been ascribed to Dolomite is present as rhombs, from a trace to a maximum magnesium enrichment associated with igneous activity of 10 percent. Detrital silt and clay constitute 20 to (rarely) (Bonatti, 1966; Riedel et al., 1961). In other cases 50 percent of unit I sediment at Sites 225 and 227. At Site dolomites are thought to have formed in association with 228 the overall detrital contribution is higher, including hypersaline brines. A DSDP core (Site 12) from the African sands of complex mineralogy as a major constituent, continental margin contained abundant dolomite and reflecting the proximity of an active Plio-Pleistocene Sudan palygorskite. Peterson, Edgar, et al. (1970) feel that Delta (see Site 228 chapter). hypersaline brines, formed in near-shore lagoons, moved Unit I sediments are generally well burrowed, but down through the deep-water sediment sequence by a occasional layering is seen as color variations, with the process of refluxion. In other cases, hypersalinity is colors being indicative of redox potential. White, brown, associated with desiccation. Miocene sediment sequences and reddish colors indicate generally oxidizing conditions, drilled in the Mediterranean contain abundant dolomite whereas the darker colors (grays, greens) indicate reducing associated with halite and anhydrite. Ryan, Hsü, et al. conditions. Darker layers contain pyrite and more abundant (1973) postulate that these sediments formed during a organic matter. "salinity crisis." Another compelling association is that of Lithified carbonate layers, sometimes rich in pteropods, dolomite and inferred reducing conditions noted in cores occur in the upper sections of unit I at Sites 225 and 227. recovered by DSDP from the African continental margin The cement, which is most often composed of aragonite and the Cariaco Trench of the Caribbean Sea (Hayes, Pimm, and sometimes of high magnesium calcite, reflects periods et al., 1973; Edgar, Saunders, et al., 1973). Whether organic at which these minerals were inorganically precipitated, matter itself is a big factor, or whether hypersalinity perhaps in conjunction with eustatic fluctuations of sea associated with environmental extremes in these inferred barred basins is controlling, is still a problem. level (or tectonic movements of a sill area). Unit II—Sediments of unit II are basically the same as Dolomite was found to constitute a significant part of those of the overlying unit except for a higher terrigenous portions of the lithostratigraphic sequences at three sites content. As in unit I, there are light gray and dark gray drilled in the Red Sea. These dolomites were studied layers, the latter enriched in pyrite and organic matter. petrographically and geochemically to determine their Unit III—This unit is a dark gray to black semilithified nature, to associate them with the depositional environ- dolomitic silty clay stone. Dolomite commonly makes up 20 ment as deduced by other means, and to see if they percent of the total, locally attaining 80 percent. The themselves could add to our knowledge of that claystone contains abundant analcite in the clay fraction, environment. 5-10 percent pyrite, and is rich (up to 6%) in organic carbon. Millerite (MS) is also present. LITHOSTRATIGRAPHY Unit IV—Unit IV is an evaporite sequence of alternating Sites 225, 227, and 228 were drilled (and continuously anhydrite and halite. The anhydrite is both of laminar and cored) in the central Red Sea Main Trough, 225 and 227 nodular texture; the halite is light gray to clear. The being to the east of the Axial Trough and 228 to the west evaporites are described in detail by Stoffers and Kuhn (this (Figure 1). The same four lithostratigraphic units were volume). Dolomite and pyrite occur in abundance as matrix recognized at all three sites. These units are fully described between anhydrite laminae and nodules and as an important constituent of black claystones which are layered within the evaporite sequence. Scripps Institution of Oceanography, La Jolla, California. Laboratorium fur Sedimentforschung, Universita^ Heidelberg, Germany. Institute of Geophysics and Planetary Physics, University of Micarb is a term used to refer to very fine carbonate particles of California, Riverside, California. uncertain origin. 867 P. SUPKO, P. STOFFERS, T. CAPLEN RED SEA 2 5' EGYPT HOT BRINE AREA 20' SUDAN SAUDI ARABIA 15' ETHIOPIA 35° 40° Figure 1. Base map showing the locations of Sites 225, 227, and 228 in the Red Sea. 868 SITE S I TE 225 227 O-i >LITHIFIED LAYERS CALCAREOUS (NANNO) OOZE and CHALK, variable DETRITAL admixture 100- CALCAREOUS SILT (STONE) and CLAY (STONE) 200- CLAYSTONE, -ich in ORGANIC MATERIAL, DOLOMITE, PYRITE EVAPORITES, with interbeds of PYRITIC UU=1 CLAYEY SILT to SAND DOLOMITIC CLAYSTONE DOLOMITE 300-1 ANHYDRITE > ö o r oo i Figure 2. Correlation of generalized lithostratigraphic sections at Sites 225, P. SUPKO, P. STOFFERS, T. COPLEN TABLE 1 coccolith plates which, by their relatively poor preserva- Unit Boundaries tion, appear to be diagenetically altering. Significant are the two round nanno(?) plates attached to the rhomb in the lower right corner of the illustration. This is taken to Sediment indicate recrystallization in process. Another such rhomb Unit Site 225 Site 227 Site 228 (same sample) is shown in Figure 4. Note here the Unit I 0-112 m 0-131 m 0-195 m (Cores 1-17) (Cores 1-17) (Cores 1-24) Unit II 112-167 m 131-194 m 195-250 m (Cores 18-22) (Cores 18-25) (Cores 24-30) Unit III 167-176 m 194-226 m 250-287 m (Core 23) (Cores 26-29) (Cores 30-35) Unit IV 176- ? m 226- 7 m 287- ? m (Cores 24-29) (Cores 30-45) (Cores 35-39) Deposition of the units at the three sites has been roughly synchronous (Figure 2), with evaporite deposition ceasing at the close of the Miocene. Unit III, the dolomitic claystone, was deposited in roughly the Early Pliocene and the biogenic deposit is subsequent to this. Since units I and II differ only in the relative amount of detrital material admixed, no further distinction will be made between them. PETROGRAPHY OF DOLOMITES Petrographic characteristics of the dolomites were Figure 4. A unit 1 dolomite rhomb lying in a matrix of studied in thin section under the petrographic microscope partially recrystallized coccoliths. Note the sliver-like (Peter Stoffers—Heidelberg) and by scanning electron particles apparently crystallizing onto the top of the microscopy (Peter R. Supko—Scripps) of slurried and fresh rhomb. Slurried sample (SEM) 225-16-2. break samples. In general, dolomites of units I and II are relatively large recrystallized coccoliths to the upper and lower right of the euhedral rhombs, while those of units II and IV are fine rhomb and the thin sliver-like particles apparently grained and anhedral. crystallizing onto the top face of the rhomb. A typical unit I rhomb is seen in Figure 3. It is euhedral Dolomites of unit II, the basal and more detrital-rich and has a crystal edge about 6µ long and is lying among section of the ooze facies, are similar and, if anything, more perfectly euhedral. Figure 5 shows dolomite euhedra lying in a matrix of clay and recrystallized carbonate. Figure 6 shows a cluster of rhombs with mutually interfering crystal faces, clearly indicating in situ growth. At higher magnification, several crystals exhibit a "spalling" or "peeling off" of crystal faces (Figure 7, note crystals in upper right center and lower left). Even if this occurred during sample preparation (this is a fresh break sample), it might still be indicative of lattice inhomogeneities. In contrast, SEM observation detected no dolomite euhedra in the half dozen unit III and IV samples studied. Figure 8 depicts a sample from the center of unit III at Site 227. Although X-ray diffraction analysis shows this sample to contain 25 percent dolomite and 75 percent detrital material, repeated scanning showed nothing other than intergrown fine anhedral crystals. An SEM photograph from the center of unit IV at this same site (227) shows a general mosaic of intergrown anhedra (Figure 8). This sample represents a dolomite-pyrite seam between anhy- drite nodules. In unit IV, dolomite is largely associated with organic Figure 3. Typical euhedral dolomite rhomb from unit I. matter and may occur as scattered anhedral crystals within Note the two round nanno (?) plates which appear to be anhydrite (Figure 9) or may be a replacement of an organic recrystallizing onto the rhomb. Slurried sample (SEM) structure, in this case an oncolite (Figure 10, see also 225-16-2. photomicrographs in Stoffers and Kuhn, this volume).